This is so fun! I like Harry Potter, I'm not a super-fan, read the books once and haven't gotten around to watching all the movies, but I love this and look forward to reading more. I hope you will consider sharing some sort of starting save file with us whenever you have it to the point where you like it.
Like @Khazthe I also thought of the club system to add more students and teachers. You have Hugo and Max and the ghost paintings (nice touch with the ghost paintings btw) so clearly you have Get Together. Why not make clubs that hangout at Hogwarts? Club size is limited but you can have I think 11 clubs in the game. With 1-2 teacher clubs and 9-10 student clubs you should have enough to make the place feel well populated, and you can choose which 1-8 sims you actually want playable to have in the household.
Oh! You could give each house a couple of student clubs, and that would give you some choices as to house-specific clothing and behavior. "Be friendly to" members of one's own house as a club rule would lead to the students having, in general, stronger friendships within houses than between houses.
